A laboratory model for the general ocean cicrulation

Abstract

A laboratory model for the study of a barotropic general ocean circulation has been constructed following the strict geometrical constraints of the B-plane approximation. Fluid is confined by plastic blocks to the volume defined by the intersection of two spherical surfaces, of common centre and slightly different radii with a circular cylinder whose axis intersects the centre of the spheres. The entire system is rotated and an interior circulation is provided by relative rotation of one of the bounding blocks. Uniform density of the rigidly enclosed fluid ensures the irrelevance of laboratory gravity and the absence of centrifugal effects on the flow. Fluid flow is observed with an electrochemical technique. Lines of coloured fluid which move with the local velocity are produced and photographed; velocities are inferred to 5% accuracy. Rossby numbers from 1.3 x 10-4 to 7.7 and Ekman numbers from 3.1 x 10-4 to 3.1 x 10-2 have been achieved. The apparatus can be oriented at an arbitrary mean latitude. The phenomena characteristic of linear subtropical gyres have been observed: a meridional Sverdrup flow, its associated zonal flow and a western boundary current. The existence, structure, and parameter dependencies of these features are in good agreement with the predictions of a general linear boundary layer analysis which has been developed for a thin barotropic ocean. The Sverdrup vorticity balance and the width and structure of the bottom frictional western boundary current have been established within the experimental uncertainties. In the nonlinear regime the position of the centre of the gyre has been measured as it migrates north-westward; a poleward eastern boundary current appears. These results agree with theoretical estimates for the onset of nonlinear behaviour. A long period time dependent flow is observed for high Rossby number. Quantitative studies have been made on an equatorial undercurrent which moves in a direction opposite to the surface forcing velocity. For low Rossby numbers this flow reverses with change in direction of the forcing velocity. In the nonlinear regime, the westward flowing undercurrent developed a streakiness and disappeared; whereas the eastward flowing current remained defined and measurable.